Rigid frame, tensioned fabric structure

ABSTRACT

A rigid frame structure having a plurality of curvilinear truss members of polygonal cross section which at their lower ends are connected to a foundation and at their upper ends are connected to at least one other truss member to form a framework. A fabric membrane covers the framework and is tensioned by cables extending between the truss members to form a stable rigid fabric covered structure.

llited States Patent [1 1 Seaman Mar. 25, 1975 1 RIGID FRAME, TENSIONED FABRIC STRUCTURE [75] Inventor:

[73] Assignee: Seaman Corporation, Millersburg,

Ohio

[22] Filed: June 20, 1973 [2]] Appl. No.: 370,028

Related U.S. Application Data [63] Continuation-impart of Ser. No. 276,899, July 31,

1972, abandoned.

Norman R. Seaman, Sarasota, Fla.

[52] U.S. C1 52/222, 52/80, 52/86, 52/63 {51] Int. Cl E04b l/347, E04b 1/32 [58] Fieldof 8 31 11 52/63, 86, 80, 83; 135/1 [56] References Cited UNITED STATES PATENTS 3,241,269 3/1966 Reffell 52/2 3,325,958 6/1967 Moore 52/63 3,417,520 12/1968 Fink 52/80 3,434,252 3/1969 Dobson... 52/63 3,465,764 9/1969 Huddle 135/1 R 3,534,750 10/1970 Kolosvary 135/1 R 3,766,573 10/1973 Burkholz 52/63 3,788,335 1/1974 Hemmelsbach 52/63 FOREIGN PATENTS OR APPLICATIONS 448,129 6/1936 United Kingdom 52/2 1,158,347 6/1958 France 135/15 CF 798,610 5/1936 France 52/86 Primary E.\'aminerFrank L. Abbott Assistant E.\'aminer-Henry Raduazo Attorney, Agent, or Firm-Hamilton, Renner & Kenner [57] ABSTRACT A rigid frame structure having a plurality of curvilinear truss members of polygonal cross section which at their lower ends are connected-to a foundation and at their upper ends are connected to at least one other truss member to form a framework. A fabric membrane covers the framework and is tensioned by cables extending between the truss members to form a stable rigid fabric covered structure.

38 Claims, 38 Drawing Figures HEAR-251975 PATENT SHEET U10F13 MR 2 5 iSTS PAIEHTEU SHEET USUF 13 PATENTEU 2 9 5 SHEET USUF 13 PATENTEUI-IIIRZS I975 3. 87

SHEET 1UIJF 13 FIG. 28

PATEHTEDHARZSIQYS SHEET 130F 13 1 RIGID FRAME, TENSIONED FABRIC STRUCTURE RELATED APPLICATIONS This application is a continuation-in-part of my prior, copending application, Ser. No. 276,899, filed July 31, 1972 and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates generally to a curvilinear rigid frame structure having a tensioned fabric covering. More specifically, the present invention relates to a rigid frame structure in which the covering is designed most effectively to accept and distribute positive and negative loading to a plurality of truss members and also to reinforce, stabilize and stiffen the frame work.

Heretofore, buildings usually have been rigid structures of wood, metal, stone, brick or concrete and generally have satisfactorily served their purpose. However, with the demands that are imposed by our constantly expanding mobile and transient society, an ever increasing need exists for portable buildings to accommodate the requirements of temporary structures. Furthermore, due to such items as exorbitant construction costs, the large amounts of capital currently required to construct a building has prompted the development of new building concepts which are intended to alleviate the financial burden.

One concept for a building that is both portable and relatively inexpensive to construct is an air supported fabric structure. Such structures generally utilize extremely strong synthetic fabrics and are inflated and tensioned by air pressure to withstand rain, wind and snow. The internal air pressure is usually maintained by a blower system consisting of one or more continuously operating fans and automatic controls. Often the structures are designed to provide a large span but with a low profile to minimize the effect of wind forces.

Although air supported structures have some advantages over standard rigid structures, they have several drawbacks. The blower and power accessories required to maintain the air pressure are subject to breakdown and power failures. The fabric cover, if not properly inflated or if subjected to high winds, may flutter and even tear. Another liability of the air supported structure is that the anchorage device must be designed to withstand the uplift created by the inflating air pressure as well as the uplift created by the flow of wind over the structure. Furthermore, although air supported structures are generally made of strong and durable fabrics, cuts and tears can occur which are difficult to repair and may even cause the structure to collapse. Another drawback is that the accumulation of localized or concentrated snow or ice is difficult to prevent and may consequently also cause collapse of the structure. Air conditioning and heating requirements tend to be difficult to satisfy because it is difficult, if not impossible, to insulate such structures so that heating and/or air conditioning of such structures becomes, therefore, quite costly. Additionally, the pendency of lights, electrical conduits, sprinkler systems, water pipes and the like from the fabric forming such a structure is not pos sible or allowable.

SUMMARY OF THE INVENTION It is, therefore, a primary object of the present invention to provide a stable, rigid frame, tensioned fabric building structure which is comparatively portable, in-

expensive and safe under extreme weather conditions.

It is another object of the present invention to provide a rigid frame, fabric covered structure, as above,

which is self-supporting and may be insulated.

It is a further object of the present invention to provide a rigid frame, fabric covered structure, as above, in which the fabric cover is not subjected to excessive tension or stress and yet is not subject to wind flutter.

It is a still further object of the present invention to provide a rigid frame, fabric covered structure, as above, in which various utility lines such as electrical and water can be suspended from the framework.

It is an even further object of the present invention to provide a rigid frame, fabric covered structure, as above, adapted for incremental increases in length by employing modular bays between end sections.

It is yet another object of the invention to provide a rigid frame, fabric covered structure, as above, which utilizes curvilinear truss members of polygonal cross section to form the framework and stringer trusses in combination with said curvilinear truss members to form the additional bays.

It is yet another object of the present invention to provide a rigid frame, fabric covered structure, as above, in which arched anchor cables are stretched between lower ends of the curvilinear truss members to apply tension to the fabric covering.

It is yet another object of the-present invention to provide a rigid frame, fabric covered structure, as above, in which the shape of the truss members, the shape of the cables and the contour of the fabric between truss members is such that the static tension at any point on the fabric cover is substantially the same for both the warp and fill strands of the fabric.

It is yet another object of the present invention to provide a rigid frame, fabric covered structure, as above, in which the rigid frame is entirely selfsupporting under all design loads and the fabric cover provides shelter as well as additional strength and safety to the structure.

It is yet another object of the present invention to provide a rigid frame, fabric covered structure, as above, in which the component parts are of such size, shape and weight as to make the structure comparatively easy to load, ship and erect.

These and other objects, together with the advantages thereof over existing and prior art forms, are accomplished by means hereinafter described and claimed.

One preferred embodiment of a structure incorporating the concept of the present invention is shown by way of example in the accompanying drawings and is described in detail without attempting to disclose all of the various forms and modifications in which the invention might be embodied, the invention being measured by the appended claims and not by the details of the specification.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a stable, rigid frame, tensioned fabric structure embodying the concept of the present invention, said Figure depicting a comparatively portable, modular building having two end sections and two intermediate bays with a portion of the fabric cover being cut away;

FIG. 2A is a side elevation of the lower portion of a truss member used to support the fabric cover of a structure embodying the concept of the present invention and depicting the substantially curvilinear shape of the lower portion of said truss member;

FIG. 2B is a side elevation of the upper portion of a truss member, the lower portion of which is depicted in FIG. 2A;

FIG. 3 is an enlarged cross section taken substantially on line 3-3 of FIG. 2A and appearing on the same sheet of drawings as FIG. 2A;

FIG. 4 is an enlarged cross section taken substantially on line 4-4 of FIG. 28 to depict the truss member center face plate and appearing on the same sheet of drawings as FIG. 28;

FIG. 5 is an enlarged side elevation of a stringer truss and head member taken substantially on line 5-5 of FIG. 1 and showing the relationship of the stringer truss and the head members to form a bay;

FIG. 6 is a top plan view of the stringer truss and head members depicted in FIG. 5;

FIG. 7 is an enlarged cross section taken substantially on line 7-7 of FIG. 5 and appearing on the same sheet of drawings as FIG. 28;

FIG. 8 is an enlarged top plan taken substantially on line 8-8 of FIG. 5 and depicting the stringer truss plate connection;

FIG. 9 is an enlarged cross section taken substantially on line 9-9 of FIG. 1 and depicting the fabric cover tensioned over the curvilinear truss;

FIG. 10 is a schematic top plan disclosing a sequen tial lateral folding prior to longitudinal rolling for bundling a fabric cover to facilitate its application to a structure embodying the concept of the present invention; 7

FIG. 11 is a top plan of the folded and rolled bundle;

FIG. 12 is a perspective view ofa section of the cover removed from between successive truss members and depicting the double curved, convoluted saddle shape assumed in response to the pretensioning of the fabric;

FIG. 13 is an enlarged elevation of the assembly by which the anchor cable on the cover is adjustably tensioned;

FIG. 14 is a reduced side elevation taken substantially on line 14-14 of FIG. 13 with the anchor cable removed;

FIG. 15 is an enlarged elevation of that portion of the rigid frame, tensioned fabric structure relating to the open area between the anchor cable and ground level and depicting one form of a curtain that may be employed to close said open area;

FIG. 16 is an enlarged cross section taken substantially on line 16-16 of FIG. 15;

FIG. 17 is a plan view depicting the anchor yoke by which that form of curtain shown in FIG. 15 may be vertically secured to a truss member;

FIG. 18 is an enlarged cross section taken substantially on line 18-18 of FIG. 15 and depicting the yoke anchored to a truss member;

FIG. 19 is an enlarged further cross section taken substantially on line 19-19 of FIG. 15 and depicting, in side elevation, a means by which the curtain shown in FIG. 15 may be secured to the ground;

FIG. 20 is an enlarged cross section taken substantially on line 20-20 of FIG. 19 and depicting the anchor plate in top plan;

FIG. 21 is a cross section taken substantially on line 21-21 of FIG. 20;

FIG. 22 is an enlarged elevational view of an alternate form of closure curtain which may be located inside of the curved bottom openings in the main cover between the bays;

FIG. 23 is an enlarged elevation of a side marginal portion of said curtain;

FIG. 24 is an enlarged elevation ofa bottom marginal portion of said curtain;

FIG. 25 is an enlarged elevation of a bottom corner portion of said curtain;

FIG. 26 is an enlarged sectional view through a side margin of the curtain showing how it is attached to the inner chords of adjacent trusses;

FIG. 27 is an enlarged sectional view showing how the bottom marginal edge of the curtain is attached to a base angle on the foundation;

FIG. 28 is a front elevation of one of the compression members extending between trusses to which the upper margin of the rectangular curtain is attached;

FIG. 29 is a partial elevation of the truss frame showing the location of the compression members, the truss structures being shown schematically;

FIG. 30 is a partial side elevation of one of the trusses showing a compression member attached thereto and the weatherseal filler panel extending between the compression member and the main cover;

FIG. 31 is an enlarged cross sectional view showing how the upper margin of the curtain is attached to the compression member and the filler panel;

FIG. 32 is a top plan sectional view on line 32-32 of FIG. 30 and appearing on the same sheet of drawings as FIG. 30;

FIG. 33 is a bottom plan sectional view taken substantially on line 33-33 of FIG. 30 and also appearing on the same sheet of drawings as FIG. 30;

FIG. 34 is a detached plan view of the filler panel;

FIG. 35 is an enlarged partial sectional view taken substantially on line 35-35 of FIG. 34',

FIG. 36 is an enlarged partial sectional view taken substantially on line 36-36 of FIG. 34; and,

FIG. 37 is an enlarged partial sectional view taken substantially on line 37-37 of FIG. 34.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, a rigid frame, tensioned fabric structure embodying the concepts of the present invention is designated generally by the numeral 10 in FIG. 1 and comprises a framework, generally indicated by the numeral 11, which supports a fabric cover, or membrane, indicated generally by the numeral 12. A static tensile stress is applied to the fabric of membrane 12 by cables 13.

Framework 11 is formed from a plurality of truss members, designated generally by the numeral 20, having a curvilinear outer, or upper, chord which supportingly engages the cover 12. Each truss member 20 is conventionally connected at its lower end to a foundation 19 and is connected at its upper end to a plurality of similar, or identical, truss members to form a polygonal enclosure. If a longer structure is desired, incremental bays may be added by utilizing one or more pairs of intermediate truss members between end sections comprising one half of a polygon having an equal number of sides, shown in Flg. 1. The curvilinear truss members 20 are generally designed to give maximum space utilization. Therefore, as best shown in Figs. 2A'and 2B,box channel, or rectangular tube, 31 forming the outer chord of truss member 20 preferably has an upright wall portion 21 which rises substantially vertically for a given distance to provide the desired wall height along the inner periphery of the structure 10. A curved portion then extends toward the center of the structure. In order to transform a desired wall height into a generally flattened roof within a desired height range and to retain the advantages inherent in a curvilinearly shaped structure, the truss member preferably has two curved portions haunch portion 22 and roof portion 23. The haunch portion 22 generally has a shorter radius of curvature than roof portion 23 so that the truss member will provide the desired interior height and span with little, or no, wasted space.

It should be appreciated that the upright portion 21 and curved portions 22 and 23 can be varied in a great number of ways to provide structures incorporating any number of desired shapes. For example, if structure is to serve as a church, wall portion 21 may be quite high and the haunch may define an extended curve of relatively long radius. If one desires to emulate a Gothic appearance, the roof portion 23 may be of relatively large radius, but if a Roman dome is to be emulated, the roof portion 23 maybe of relatively short radius.

If an exhibit structure is desired, as might be used at a fair, upright portion 21 may be of moderate height and roof portion 23 may have a long span. Depending upon the type of displays to be exhibited, the haunch portion 22 may be fairly sharply curved to achieve only moderate roof height or the haunch portion may be gently curved so that tall displays may be housed within the structure 10. It should be appreciated that portion 21 may also be curved to effect the desired interior configuration of structure 10.

Although thetruss member is not limited to three portions, in order to achieve a desirable span and roof elevation the truss member 20 will generally have three portions with at least two the haunch and roof portions being curved. When so constructed the haunch portion 22 will generally have a sharper degree of curvature than roof portion 23. Because the truss members 20 are substantially curvilinear, they may be parabolic, sinusoidal, arcual or any composite curvilinear shape. In the specific embodiment shown two portions are curved and both portions are arcual. That is, portion 22 extends approximately 12 feet along a chord from point B to point C (FIGS. 2A and 9) and has a radius of curvature of l2 feet (subtending a horizontal span of slightly more than 4 feet), and portion 23 extends approximately 32 feet along a chord from C to D (FIG. 9) and has a radius of curvature of 49 feet (subtending a horizontal span of slightly more than 30 feet). Vertical portion 21, which extends from point A to point B, is depicted as being approximately 6 feet in height.

Considering now the components of the truss member 20, as aforementioned it has an outer, curvilinear box channel 31 comprising the upper, or outer, chord and a series of parallel, connected, straight structural components that may be fabricated from angle iron and which comprise the lower, or inner, chord sections 33a, 33b, 33c and 33d. As best seen in FIGs. 2A and 2B, the lower chord sections follow the general outline of the curvilinear box channel 31 comprising the outer chord.

cated from angle irons and which connect box channel 31 to chord components 33 along the length of the truss member. Truss members 20 are interconnected to form framework 11. These connections employ one or more variations of a face plate 35, as is hereinafter more fully explained.

The cross section of truss member 20 generally is of any conventional polygonal shape or may simply be an- I-beam. As shown in FIG. 3, truss member 20 is preferably of triangular cross section, because this shape has been found to reduce the amount of steel required and to provide much greater transverse strength than an I- beam of equal weight, and thus is not only economical but also provides sufficient lateral stability so that the truss member will not bend laterally, or twist, as it is being handled during erection of structure 10. Additionally, the triangular cross section provides sufficient resistance to torsional loads on the trusses assembled from truss members 20 that shear stresses may generally be fully balanced between consecutive trusses by means of light cross rod bracing 18 (FIG. 29) in the plane of the lower chord components 33. This position for the bracing allows ample room for the fabric cover to assume the hereinafter defined saddle shape, does not interfere with the head room within the structure and eliminates the necessity to puncture the fabric cover as would be necessary for normal cross bracing. Moreover, the cross rod bracing may be designed to support the fabric skin in the possible event of a complete skin failure. In the end sections of the building a single horizontal rod that extends between the intersections of chords 33b and 33c i.e., the field joint on adjacent truss members is generally all that is required to resist torsional stresses to the trusses.

To satisfy special environmental conditions and/or with particular embodiments it may well be required to provide additional cross bracing between adjacent truss members at one or more locations within the structure.

The base width of the truss member 20, as with most trusses of polygonal cross section, is calculated by conventional methods to withstand the application of normal transverse loading forces. Excessive base widths are generally not used because they unnecessarily increase the weight and the cost of the truss.

Sizing of the structural components for the truss members is determined in accordance with conventional engineering techniques as known to one skilled in the art. Briefly, after the height of wall portion 21 and the lengths and radii of curvatures for the haunch portion 22 and roof portion 23 have been selected in a manner as described hereinbelow, the various loads, including wind load, are then determined. With these assumed loading conditions, stress analysis of the structure is conducted in order to determine the minimum size requirements, including a reasonable factor of safety, of each specific truss component.

The basic horizontal shape of framework 11 is a polygon. However, when additional space is desired, bays 15 may be added between the end sections of structure 10 by the incorporation of one or more pairs of opposed truss members 20 spaced laterally between end sections. In order to connect these pairs of bay truss members with each other and/or to the structural end sections, a stringer truss, generally indicated by the numeral 40, is provided. For most installations only one stringer truss per bay section is needed, and it usually connects the mid-point of each pair of opposed truss members to the mid-point of another pair of laterally spaced, opposed truss members or to the convergence of the truss members in the end section.

The cross-sectional shape of the stringer truss, as with the curvilinear truss members, is generally polygonal to provide transverse stability and, as seen in FIG. 7, is preferably triangular inasmuch as such a shape favorably satisfies considerations of weight, size, cost and strength. Stringer truss 40 is generally similar to curvi linear truss in that it has a box channel or rectangular tube 41 vertically spaced from parallel chord members 43a and 43b that are themselves laterally spaced by bars 42. Reinforcing struts 44 also interconnect the box channel 41 with the two chord members 43a and 43!) throughout the length of the stringer truss 40. Any type of structural components may be used, although angle iron is often preferred because that shape is relatively inexpensive and because that shape affords favorable strength and bending characteristics. As best seen in FIG. 5, stringer truss 40, unlike curvilinear truss 20, is straight and inverted so that box channel 41 comprises the lower chord member. However, the ends of stringer truss 40 are inclined laterally so that the intermediate portion is offset from its ends in order to permit the tensioned fabric cover 12 to form a saddle between adjacent pairs of bay truss members and between the bay truss members in the end section of the framework. The amount of offset is preferably not much greater than the maximum depth desired for the saddle formed by the membrane of the fabric cover extending between the successive truss members in the framework inasmuch as an excessive offset would complicate the structure required to maintain the lateral stability desired for the stringer.

Sizing of the various stringer truss components will generally depend only upon the horizontal forces acting on structure 10. In accordance with conventional practice as known to those skilled in the art, the various loads, including wind, are determined, and after applying a factor of safety the tensile or compressive stress resulting in each of the specific structural components is calculated. The type and size of each component is then selected.

In the assembled framework 11, truss members 20 are preferably connected to one another at their upper ends in the manner depicted in FIGS. 1, 5, 6 and 8, wherein the various truss members are bolted through face plates 35 to a polygonal head member, designated generally by the numeral 50. Head member 50 is designed to transfer loads between truss members, to allow sufficient opening for gravity or forced air ventilation and to provide a weather seal.

Generally, head member 50 has six sides or faces 51 because a structure of regular hexagonal shape requiring six truss members has been found to provide a very stable structure and to permit the fabric cover 12 to be tensioned substantially equally along both the warp and fill strands. If the use ofa greater number of truss members is desired, the head member may have eight, ten or more sides provided that there are preferably an even number of sides to facilitate the incorporation of bay sections. Head members 50 may also be used to connect the opposed bay truss members to each other and to the stringer trusses. For most structures the stringer trusses are desirably aligned across the head member 50 so that with the bay truss members connected to the opposed flat surfaces of a hexagonal head member 50, the inclined ends of the stringer truss are provided with V-shaped face plates in order to receive an apex 52 of the head member and effect a stable joinder therewith.

To erect the framework 11 a curvilinear truss member 20 is connected to a head member and raised into position. Then the base of that truss member is anchored to a foundation according to any conventional manner. Thereafter three additional truss members are positioned and connected to that same head member 50, and they too are anchored to appropriate foundation piers. I

Even though only the structure of an end section is so formed, it is self-supporting and thereby greatly facilitates completion of the structural framework. If a structure does not have intermediate bays, the remaining truss members can similarly be positioned and connected to form a polygonal framework. If the structure is to contain intermediate bays, the desired number of pairs of truss members are positioned and connected by stringer trusses sequentially from the first head member 50 and then the truss members forming the remaining half polygonal end section are securely positioned to produce a rigid frame structure such as shown in FIG. 1. As with other rigid frame structures, various items including utility lines, sprinkler lines and the like can be supported from the framework. Furthermore, the truss support chords 33 provide an ideal structural member to which insulation and/or a decorative interior surface can be attached.

Framework 11 supports a fabric membrane, or cover, 12 to complete the basic enclosed structure 10. After the framework 11 is fully erected the cover 12 may be applied. To facilitate positioning of the cover it may be successively folded and rolled into a compact bundle, placed at the top center of the framework and successively unrolled and unfolded into position.

One convenient arrangement for bundling the cover, as shown in FIGS. 10 and 11, is accomplished by folding the lateral sides to the longitudinal center line thereof (one side so folded is depicted in FIG. 10) and then rolling each longitudinal end of the folded cover individually toward the center of the folded cover with a lading bar 52 centered within each rolled end. The resulting bundle 53 may then be strapped, as at 54, to maintain its configuration. The ends of the lading bars 52 project laterally from the resulting bundle to provide a means for lifting the bundle from place to place and to the top centerline of the framework 11 in which position the strapping 54 may be removed and the lading bars 52 used to assist in unrolling the folded cover along the centerline of the framework. After being unrolled the sides may be unfolded outwardly and downwardly along the framework conveniently to effect an approximate positioning of the cover on the framework.

Cover 12 is so designed, as described below, that it can be statically prestressed in tension substantially equally along both the warp and fill strands at any given point and is preferably made from a fabric which maintains dimensional stability over a wide range of temperatures and humidity. Although several fabrics may be used, a polyester fabric such as Duponts Dacron has been found to have these desirable properties. Additionally favorable properties of polyester over other types of fabric such as nylon include far superior ultraviolet light resistance, the ability to withstand weather exposure over long periods of time, and a high tear strength. In order to balance, as nearly as possible, the amount of initial stretch available in the warp and fill strands, a knitted fabric may be utilized to advantage. Preferably, the fabric is comprised of essentially straight warp and fill strands disposed at substantially right angles with respect to each other and knitted together by a third yarn system which allows both the warp and fill to remain relatively straight and does not impart the crimp to the fill yarns occasioned by nonknitting weave techniques. Such a fabric structure thereby tends to equalize the amount of stretch between the warp and fill yarns induced by crimping the fill strands during the weaving process.

Although the inequality in the amount of stretch between warp and fill strands occasioned by the crimp normally imparted to the fill strands during weaving is greatly minimized by knitting the base cloth, the lineal stretch characteristics are not altogether eliminated because even the handling of the fabric during the coating process applies a certain tension to the warp strands, thereby increasing, to a modest degree, the amount of stretch available to the fill strands as compared to the warp strands.

In order to prevent water from penetrating the fabric membrane, as well as to impart mildew, ultraviolet and abrasion resistance, the membrane is preferably coated with any high quality vinyl compound by techniques well known to those skilled in the art.

Considering now the special manner in which the membrane is applied to the structure so that substantially uniform tension exists throughout the membrane, it is of primary importance that fluttering due usually to windlift i.e., the application of subatmospheric pressure to the exterior surface of the membrane be minimized. To avoid flutter, the contour given to the fabric must not be essentially flat or located in a single curved plane. A saddle shape has been found to be effective in withstanding both positive and negative (windlift or minus pressure) loading. In order to achieve this contour, forces must be applied to the membrane; that is, the membrane must be pre-loaded in tension over the framework.

In orienting the fabric membrane it is desirable that the warp and fill strands be aligned respectively with the longitudinal and the generally vertical axes of the structure so that the warp strands (minimum stretch) will support positive loading (including loads of longer duration, such as snow and ice) and the fill strands will resist the negative loading (fluctuating wind lift). This orientation permits the positive live loads to be transferred to the trusses with the least deformation of the outer membrane.

As previously noted, the height and width of structural framework 11 is chosen to give the desired inside space. However, these parameters are chosen with a practical eye toward truss construction as understood by one skilled in the art so that the required depth of the truss member, particularly at haunch portion 22, is not overly large or overly expensive. Moreover, the radius of curvature of the haunch portion must be kept within practical limits so that the tension of the fabric membrane between adjacent haunch portions is not so great as to put undue stress on the fabric. For example, in the particular embodiment shown in FIG. 2, the approximately 6 foot upright portion 21 of box channel 31 achieves a substantial wall height which then gives way to a haunch portion the outer curve of which is struck by a 12 foot radius and extends for approximately ten feet, as measured along the chord of the arc. From a practical standpoint this curvature has been found not to be sufficiently sharp as to necessitate overly heavy, or excessively expensive, truss components to support the roof portion. Moreover, undue stress on the covering fabric may be avoided because the haunch curvature is thus moderate.

Compatibly with the aforementioned haunch configuration the roof portion 23 may employ a radius ofcurvature of 49 feet and extend for approximately 30 feet. as measured along the chord of the arc. When two such truss members are opposedly connected to a head member 50, the resulting structure provides a 68 foot clear span without interior supports. By locating one pair of opposed truss members medially of the end portions, as shown in FlG. l, and 20 feet from each end portion the resulting two bay structure provides over 6,300 square feet of unobstructed floor space. By adding one additional pair of opposed truss members that are also 20 feet on center from the adjacent truss members the resulting three bay structure provides approximately 7700 square feet of unobstructed floor space.

Having selected a fabric well suited for the cover and having determined the proper orientation for the warp and fill yarns, as well as the general height and width of the structure, the fabric cover is designed so that when stretched over the framework 1], the warp and fill strands of the cover will be pretensioned to approximately equal tensile stress at all points along the curved surface thereof. Perfect uniformity in matching the tensile stresses in the warp strands with the tensile stresses in the fill strands throughout the cover is, however, substantially impossible to achieve because of the varying curvature of the cover 12. It must be appreciated that the tensile stress in either a warp or fill strand at any given point on the cover equals the product of the load at that point multiplied by the radius of a curve along which the particular strand lies at that point.

As can be seen by reference to FIG. 12, the fabric cover 12 assumes a double curved surface between any two consecutive truss members. This double curved, or convoluted, disposition of the fabric is designated as the saddle configuration, and the saddle seat S refers to a reference line on the fabric cover 12 which lies in a plane centered between any two consecutive truss members. The disposition of any warp strand W delineates the warp curve, and the warp curve constitutes that direction on the fabric cover which spans any two consecutive truss members and lies in a plane which passes through the intersection of that line and the saddle seat S and which plane is perpendicular to a line tangent to the saddle seat S at the point of intersection. The radius R, of the warp curve lies within that plane.

The disposition of any fill strand F delineates the fill curve, and the fill curve constitutes that direction on the fabric cover 12 which lies between two consecutive truss members and also lies in a plane that is parallel to the plane which includes the saddle seat S. The radius R; of the fill curve lies within the plane of the fill curve.

With respect to the structure 10, the warp radius R lies exteriorly thereof, and the fill radius R; lies interiorly thereof.

Briefly, the procedure for designing that portion of the fabric cover intended to span a bay section is as follows. Starting at the apex, or ridgerow of the structure, a trial fill radius R is selected which in order that the saddle seat S will not be designed to lie radially outwardly of the curved outer chord defined by box channel 31 is necessarily less than the greatest radius of the truss member. In order to design toward the desired stress balance between warp and fill strands the dimension of the trial fill radius R, so selected is then considered as the dimension for the trial warp radius R and in this context the dimension must be large enough so that the saddle seat S will clear the stringer truss 40 sufficiently to allow for stretch of the cover 12 due to snow or ice loads. As the next step in designing the fabric cover, a second trial radius is selected (for the portion of the cover lying between the centers of the haunch portions of adjacent trusses). The dimension of this second trial radius must be large enough so that when applied as the warp radius the resulting saddle seat will lie outside the planes of the lower truss chords 33. This second trial radius must also be small enough so that when applied as a fill radius one end of the resulting saddle seat between the haunch portions of consecutive truss members will intersect the saddle seat defined by the first trial radius along a common tangent and the other end of the resulting saddle seat between the haunch portions will intersect the point of anchorage along a tangent to the saddle seat.

The entire saddle seat resulting from the two trial radii is somewhat similar to the outside member of the truss in that it consists of a short, straight section that extends upwardly from the point of anchorage and thereabove extends, by successive, merging arcs having two radii, to the apex, or ridgerow of structure 10. However, the fill radii of the cover more nearly approach each other than the radii of the truss members 20. in the particular embodiment depicted, the fill radii of the cover are about 45 feet and 22.5 feet, respectively. By interpolation, this trial saddle seat is modified to a somewhat parabolic curve such that the warp and fill radii at any point along the modified saddle seat are as nearly equal as possible.

The cover for a polygonal, or end, section is designed with radii similar to those used in designing the bay sections so as to maintain equal tensions on either side of a truss member at any point along the truss member. However, in an end section, the saddle seat actually extends to the apex rather than to a level below that of the apex, as in a bay.

According to expected field conditions, strength and stretch of the fabric under design loads, including both wind force (positive) and wind lift (negative as well as any snow loads, are calculated according to methods well known to one skilled in the art. Using these loads, and, of course, a factor of safety, the tension on the cover is calculated for that portion in proximity to the head member 50 as well as that portion adjacent the haunches to determine if the stresses in the cover resulting from the expected maximum loading is reasonably well below the fabric strength. If it is not, inasmuch as the load multiplied by the radius of curvature for either the warp or fill strands equals the tensile load in that strand, the radius of curvature may be reduced to reduce the tension.

By using test data which reveals the percent of stretch in the fabric under various loads along both the warp and fill directions (each react differently) the clearance between the fabric and the stringer trusses may be calculated for expected load conditions. If the calculations determine that contact may result. this condition may be obviated in any of a number of ways. For example, one may position the truss members delineating the bays slightly closer to each other, thus allowing a reduced fabric radius of curvature, or one may increase the dimension to which the stringer is offset. Using the particular embodiment shown in the drawings as an example, the fabric tension at maximum load conditions has been calculated to be approximately pounds per inch at the head member portion and approximately 45 pounds per inch at the haunch portions. This is fully within the strength of available fab- I'lCS.

Having thus determined suitable radii and curves to be used in the cover design, and having determined that the fabric covering can withstand the maximum anticipated loading, it is a matter of geometry and trigonometry to develop patterns for the generally trapezoidal panel sections which, when sewed or welded together, should form a complete cover. However, because fabrics are not fully dimensionally stable, it is necessary to predetermine the amount of pre-loading and reduce the patterns by the amount of stretch that will be developed in both the warp and fill strands as a result of the pre-loading.

For the particular embodiment depicted herein, a maximum pre-loading tension of approximately l0.8 pounds per inch may be chosen. This maximum will occur in that portion of the cover located medially of the haunch portions in adjacent truss members and will develop a minimum tension of about 5.4 to about 6.3 pounds per inch in that portion of the cover located medially the apices of adjacent truss members i.e., in proximity to the head member 50. These tensions are deemed to be substantially equal in view of the fact that the tear strength of the fabric is at least ten times the maximum pre-loading tension.

Because the application of positive loads relax negative pre-loads and because the application of negative loads relax positive pre-loads, in a building embodying the concept of the present invention, positive and negative loading need not be considered as being cumulative.

Actually, the maximum pre-load is limited by the stretch characteristics of the fabric. A fabric cover having a low stretch factor, after having been reduced by the stretch developed by a very light pre-load, thus would be very difficult to pull over the rigid frame.

Briefly, the manner by which the particular shape for the fabric panel sections is determined will now be described. The length of any fabric section is such that when placed on framework 11 longitudinally thereof and stretched to impart the desired pre-load tension, the negative radius of curvature i.e., the warp radius R will approximately equal the positive radius of curvature i.e., the fill radius R and thereby achieve the theoretically required material disposition the saddle shape. Because the fill radius of curvature changes in going from the level of the haunch portions to the level of the head portions, the negative warp ra- 

1. A tensioned fabric structure having a rigid frame comprising, a plurality of curvilinear truss members forming a domed framework and each having a generally polygonal cross section, each said truss member being supported at its lower end on a foundation and connected at its upper end to at least one other of said truss members, each said truss member having a lower upright curved haunch portion connecting with a relatively flattened upper curved portion, a fabric membrane supported on and covering over said framework, said membrane having warp yarns running horizontally and fill yarns running vertically of said structure, and cable means extending between the bases of said truss members and attached to said membrane between said truss members for tensioning said membrane to form a stable structure.
 2. A tensioned fabric structure as in claim 1, in which the fabric membrane has a substantially saddle shape between truss members.
 3. A tensioned fabric structure as in claim 1, wherein the truss members are substantially triangular in cross section, said triangular cross section having an apex engaging said fabric membrane.
 4. A tensioned fabric structure as in claim 1, wherein a stringer truss is connected between the upper ends of spaced pairs of said curvilinear truss members to form a bay section.
 5. A tensioned fabric structure as in claim 4, in which the stringer truss has its intermediate portion bodily offset from its ends.
 6. A tensioned fabric structure as in claim 4, in which the stringer truss has a triangular shaped cross section, said triangular cross section having an apex directed toward the interior of said structure.
 7. A tensioned fabric structure as in claim 6, in which the stringer truss has its intermediate portion bodily offset from its ends.
 8. A tensioned fabric structure as in claim 1, wherein the truss members have inner and outer chords and each cable means tensioning said membrane defines an arcual path in a vertical plane forming a curved opening at the bottom of said membrane.
 9. A tensioned fabric structure as in claim 8, in which said fabric has a substantially saddle shape between truss members and said cable means conforms to said saddle shape.
 10. A tensioned fabric structure as in claim 1, wherein each cable means tensioning said membrane defines an arcual path projected in a vertical plane.
 11. A tensioned fabric structure as in claim 4, wherein said fabric membrane has a first radius of curvature at the apex in a vertical plane perpendicular to the curvilinear truss members delineating said bay sections, said radius being less than the largest radius of the truss members delineating said bay sections.
 12. A tensioned fabric structure as in claim 4, wherein said fabric membrane has a second radius of curvature in a plane perpendicular not only to the truss members but also to a second plane tangential to the center of the haunch curves, said radius being larger than the radius of curvature of the haunch portions of said truss members.
 13. A tensioned fabric structure as in claim 8, wherein the curved bottom openings formed in the fabric between truss members by each cable means are closed by a planar fabric curtain having an upper and lower margin and extending between the inner chords of adjacent truss members and upwardly from the foundation, the upper margin of said curtain being secured to a compression member extending between the inner chords of adjacent truss members.
 14. A tensioned fabric structure as in claim 13, in which a fabric filler panel extends outwardly from said compression member and seals against the inner surface of said fabric membrane covering.
 15. A tensioned fabric structure as in claim 13, in which tensioning means connect said curtain to the inner chords of adjacent truss members and to the compression member.
 16. A tensioned fabric structure as in claim 14, in which tensioning means connect said curtain to the inner chords of adjacent truss members and to the compression member.
 17. A tensioned fabric structure as in claim 14, in which said fabric filler panel is of bias cut fabric having its inner edge longitudinally pretensioned and its outer edge adapted to seal against said fabric membrane covering when longitudinally tensioned.
 18. A tensioned fabric structure as in claim 17, in which means is provided to attach the inner edge of the filler panel to said compression member, and means are provided at the ends of said filler panel to apply and maintain tension in its outer margin.
 19. A tensioned fabric structure as in claim 17, in which said filler panel has laTeral stiffener bars encased therein.
 20. A tensioned fabric structure as in claim 17, in which the means to apply and maintain tension in the outer edge of said filler panel are bars encased in the ends of said panel and adapted to be secured to said adjacent truss members.
 21. A tensioned fabric structure as in claim 8, wherein the curved bottom openings formed in the fabric between truss members by each cable means are closed by an underlapping fabric curtain extending between at least two adjacent truss members and upwardly from the foundation beyond the upper extent of the curved bottom openings, said curtain having fabric channel brackets secured to said curtain and fitting over the outer chords of said truss members for holding said curtain against lateral movement, and fabric anchor yokes connecting said brackets to the truss members to hold the curtain against vertical movement.
 22. A tensioned fabric structure as in claim 21, in which an anchoring cable extends along the bottom edge of said curtain, and tensioning means is provided to connect said cable at intervals to the foundation.
 23. A tensioned fabric structure as in claim 21, in which said fabric anchor yoke has a tab secured to one of said fabric channel brackets and two arms having inner and outer ends extending laterally around the outer chord of a truss member, and a retainer pin securing the outer ends of said arms to said truss member.
 24. A tensioned fabric structure as in claim 22, in which said fabric anchor yoke has a tab secured to one of said fabric channel brackets and two arms extending laterally around the outer chord of a truss member, and a retainer pin securing the outer ends of said arms to said truss member.
 25. A tensioned fabric structure as in claim 22, wherein the tensioning means comprises a link chain secured to said cable means, an anchor means, a clamp means having a hook portion and a head portion, said head portion being selectively interconnected to said anchor means, said hook portion engaging a selected link in said chain.
 26. A tensioned fabric structure as in claim 25, wherein means are provided for adjustably positioning said hook portion with respect to said anchor means.
 27. A tensioned fabric structure as in claim 25, wherein a series of link chains are secured to the cable means attached to said membrane, a series of second anchor means are provided and second clamp means having a hook portion and head portion interconnect the chains in said series to said second anchor means, said hook portion in said second clamp means engaging a selected link in said series of chains and the head portion of said second clamp means being selectively interconnected to said second anchor means.
 28. A tensioned fabric structure as in claim 25, wherein said anchor means comprises a plate having at least one T-slot therein and said head portion presents a stud selectively insertable through said T-slot and engageable behind said plate.
 29. A tensioned fabric structure as in claim 1, wherein certain of said truss members form polygonal end sections viewed in plan.
 30. A tensioned fabric structure as in claim 1, wherein certain of said truss members form polygonal end sections viewed in plan and at least one bay having an upper end is formed between the end sections.
 31. A tensioned fabric structure as in claim 29, wherein a head member is connected to the upper ends of said truss members, said head member affording a vent opening.
 32. A tensioned fabric structure as in claim 30, wherein a stringer truss spans the upper end of the bay, and a head member is connected to one end of the stringer truss and to the upper ends of the truss members in the adjoining end section.
 33. A tensioned fabric structure as in claim 1, wherein said fabric membrane, when removed from the framework, may be bundled securely onto a pair of lading bars, said membrane having lateral sides, said lateral sides being folded to the longitudinal centerline of said memBrane to present longitudinally opposed ends, the opposed ends of said folded cover being rolled about said lading bars, and strap means securing the resulting bundle.
 34. A tensioned fabric structure as in claim 1, wherein an adjustable tensioning assembly is provided to connect the cable means to said truss members.
 35. A tensioned fabric structure as in claim 34, wherein said tensioning assembly comprises ear means to which the cable means extending to the truss member at which said tensioning assembly is located are connected, a plurality of fixed notches presented from said truss member, an anchor bar secured to said ear means and receivable in selected notches.
 36. A tensioned fabric structure as in claim 35, wherein a loop is presented from said tensioning assembly and opposing lugs are presented from said truss members in spaced relation with respect to said loop.
 37. A tensioned fabric structure having a rigid frame comprising, a plurality of curvilinear truss members forming a domed framework and each having a generally polygonal cross section, each said truss member being supported at its lower end on a foundation and connected at its upper end to at least one other of said truss members, each truss member having a lower upright haunch portion connecting with a relatively flattened upper portion, a fabric membrane supported on and covering over said framework and having a substantially saddle shape between truss members, cable means extending between the bases of said truss members and attached to said membrane between said truss members for tensioning said membrane, the saddle shape of the membrane being curved in directions longitudinally and laterally of said truss members, and the radii of said curvatures being so related to the radii of curvatures of the truss members that the applied tension at any point of said membrane is substantially equal in directions longitudinally and laterally of said truss members to form a stable structure.
 38. A tensioned fabric structure as in claim 37, wherein the lower portions of said truss members each extend substantially upright and then curve inwardly to form a haunch portion which connects with a relatively flattened curved upper portion. 